In many communications applications it is advantageous and sometimes even necessary that protocol data units are defined to belong to different traffic categories such as Classes of Service (CoS) based on the requirements of the applications using the communications service and, on the other hand, on the Service Level Agreements (SLA), for example, between the service provider and the customers. Such protocol data units may be e.g. IP (Internet Protocol) packets, ATM (Asynchronous Transfer Mode) cells, Ethernet units, Frame Relay units, or MPLS (Multiprotocol Label Switching) units. For example, in a VoIP (Voice over Internet Protocol) telephony application it is important that the data transfer delay and delay variation remain below permitted limit values. Such permissible limit values can be defined in a service quality agreement, for instance. However, when downloading a web page, for example, the transfer delay and its variation are significantly less critical quantities.
It is often desirable to monitor and/or limit the speed properties of a traffic flow comprised of protocol data units to be transferred. A speed property may refer to the Committed Information Rate (CIR), Committed Burst Size (CBS) by which the CIR can be momentarily exceeded, or the Peak Information Rate (PIR), for example. A traffic flow may include protocol data units representing different traffic categories such as different classes of service, for example. Such a situation occurs, for example, if it is desired to reserve to a certain customer a specific portion of the data transfer capacity [bit/s] of a data transfer link so that the customer's traffic will not be permitted to exceed the transfer capacity reserved to that customer. The various protocol data units of the traffic flow representing the traffic of that particular customer may belong to different traffic categories such as classes of service, for instance. Part of the customer's traffic may be delay-critical transfer of voice or video signal, for instance, and part of it may be non-delay-critical file transfer, for instance. Monitoring and/or limiting of the speed properties of a traffic flow should not increase the transfer delay nor the variation of the delay of delay-critical traffic.
The speed properties of a traffic flow typically are monitored and/or limited at a network element of a communications network, which network element is adapted to receive, buffer and forward protocol data units. The network element may be e.g. an IP (Internet Protocol) router, Ethernet switch, ATM (Asynchronous Transfer Mode) switch, base station of a mobile communications network, an MPLS (Multi Protocol Label Switching) switch, or a combination of these. The speed properties typically are monitored and/or limited by monitoring the arrival speeds of traffic flows arriving at the network element and/or by monitoring and/or shaping the transmission speeds of traffic flows departing from the network element. The operation in which the speed properties of a traffic flow are altered is referred to as ‘shaping’, and the equipment that does the shaping is referred to as a ‘shaper’.
Publication US2007/0002741 discloses a method and equipment for shaping the transmission speed of a traffic flow when the traffic flow also contains delay-critical data traffic. In connection with the sending of each protocol data unit it is determined whether the sending of the protocol data unit to be sent next has to be delayed in order to meet a condition set on the transmission speed. Thus the need for delay is determined regardless of whether the protocol data unit which is to be sent (or was sent) is representative of delay-critical or non-delay-critical traffic. The sending of the protocol data unit to be sent next is delayed only if there is a need for delay and if the protocol data unit to be sent next is representative of non-delay-critical traffic. So, the sending of protocol data units representing delay-critical traffic will not be delayed even if there were a specified need for delay. This way the transfer speed of non-delay-critical traffic will adapt to variations of transfer speed of delay-critical traffic and, on the other hand, the shaping of the transmission speed will not increase the transfer delay nor variation of the delay of delay-critical traffic. Transmission speed of a traffic flow can be limited according to desired speed and/or burst limitations if the portion of delay-critical traffic in the traffic flow is so small that the delay-critical traffic alone will not exceed the speed and/or burst limitations. If the delay-critical traffic exceeds the speed and/or burst limitations the transmission speed of a traffic flow cannot be limited according to the speed and/or burst limitations because the sending of protocol data units representing delay-critical traffic shall not be delayed even if there were a specified need for delay. Delaying the sending of protocol data units representative of delay-critical traffic would increase the transfer delay and variation of delay of delay-critical traffic.